1. Field of the Invention
The present invention relates generally to an ignition control system and more specifically to a ignition control system wherein the crank angle level change from a predetermined crank angle position is determined using a basic control signal and which estimates the delay period after which the ignition should be implemented.
2. Description of the Prior Art
Electronic engine control system determines the ignition timing (crank angle) in accordance with engine operating parameters and output an ignition control signal at the calculated timing.
In order to achieve this control it is usual to use a crank angle sensor arrangement which generates a basic control signal or signals and a series of timing signals at 1.degree. or 2.degree. during the intervals between the generation of the basic control ones. The control system initiates engine ingition a predetermined number of timing signals have been detected as being generated after a basic control signal.
However, this technique requires that the crank angle sensor exhibit a very high degree of accuracy. Achieving this accuracy of course increases the cost of the sensor considerably.
To overcome this problem it has been proposed JP-A-61-285584 to use a technique wherein the crank angle sensor is arranged to produce only the basic control signal or signals and to estimate via calculation, the time after the generation of basic control pulse or signal that an ignition command signal should be generated.
However, this technique has suffered from the drawback that during transitional engine operation such as acceleration, deceleration etc. For example, during acceleration wherein the engine rotational speed increases, the ignition advance tends to diverge from the appropriate timing in a manner which tends to result in after-burn, power loss, misfiring etc, which lead to the increase of CO and HC emissions and the amount of fuel which is consumed. On the other hand, during deceleration, as the engine speed reduces the ignition retardation tends to deviate in a manner which invites misfire and engine knocking.
Therefore, when the above type of crank angle sensor is used, it has been suggested that in addition to the control pulse generation period, the period variation rate (the deviation between the instant period and the immediately previous one) be derived and used in combination with the period.
That is to say, as shown in FIG. 1, the crank angle sensor is such that the leading and trailing edges of a basic control signal REF are arranged to be produced respectively at 75.degree. ad 5.degree. before TDC of the compression stroke of each cylinder.
The time period ADVT defined between the point in time at which the trailing edge of the REF pulse occurs and the point in time at which the appropriate ignition timing ADV occurs, is derived and a timer used to count from the generation of the REF pulse to the time at which the ignition control signal should be issued.
More specifically, the above ADVT value is derived using the following equation: ##EQU1##
The first part of this equation defines a proportional value which is related to the REF period. Viz., if the engine speed is constant, the first part of the equation represents a proportional valve which is based an interval TREFL defined between the trailing edge of the previous REF pulse and the leading edge of the instant REF pulse (hereinafter referred to as the proportional control valve). The second part of equation (1) involves the rate of change of the REF period. Viz., this part of the equation makes use of the difference between a value TREFO (defined between trailing edges of the REF pulse and a value TREF defined between the leading edges of the REF pulse, as modified by a proportional constant K. Thus, during the time the engine speed increases, according to the amount by which the rate of change increases, the ignition timing is advanced. On the other hand, during the time the engine speed is decreasing the ignition timing is retarded in accordance with the amount by which the rate of change reduces. Hereinafter the second part of the equation will be referred to as the anticipation control value.
However, even when this type of technique is employed in combination with the above mentioned type of simplified rotational angle or crank angle sensor in order to avoid the cost increasing precision requirements, still the following problems are encountered.
For example, if as shown in FIG. 1, during the period a the crank angle speed increases while in period b the crank angle speed decreases, an error in the retardation is introduced into the proportional control factor. Thus, even though the anticipation control value is provided, as the crank angle speed is changing in the above described manner, an error in the anticipated degree of advance is induced. This in effect increased the magnitude of the error and results in the amount of erroneous advancement being markedly increased.
Further, as the proportional control valve is derived using the value TREFL which is defined between the trailing edge of the previous REF pulse and the leading edge of the instant REF pulse and the anticipation control value is estimated based on the rate of change between the previous period TREFO and the instant TREF period, the above mentioned error is amplified to the degree that misfire and knock tend to be promoted.
In view of the above problem, it has been thought to calculate the above mentioned ADVT period with respect to the trailing edge of the REF pulse, however depending on the engine operating conditions it sometimes happens that by the time the calculation process has been completed, the proper ignition timing has already been passed and the ignition timing is shifted to the point of deteriorating the combustion characteristics.